The present application is directed to a center line threaded cage with a winged end cap for implantation between a pair of adjacent vertebrae in order to provide spacing, orientation, and support to the vertebrae and to promote fusion between the vertebrae.
In the human spine, the pad or disc between vertebrae is sometimes damaged or deteriorates due to age, disease, injury, or congenital defect. The vertebrae may also become compressed or otherwise damaged. Because of this, surgery is often utilized to place spacers or interbody devices between the vertebrae which provide proper spacing of the vertebrae and which also promote fusion between the vertebrae. When a device of this type is utilized for purposes of promoting fusion, it is often referred to as a fusion cage or an intervertebral fusion device. When utilized to promote fusion, the interbody devices often are windowed and packed with bone fusion material to promote growth of the bone between the vertebrae. Sometimes such material is packed between a pair of devices that are placed in close proximity to one another between the vertebrae to promote growth of bone and, therefore, fusion between the vertebrae.
In the past, interbody devices have typically been either generally rectangular or cylindrical in shape. The cylindrical devices have an advantage that they can be threadably received more or less directly between and into the adjacent vertebrae. For this purpose, the vertebrae are typically first spaced apart, and then a tool is utilized to create a partial bore in each vertebra which with spacing of the vertebrae allows the interbody device to be received between the vertebrae. Because of the natural space between the bones, the interbody device usually engages the bones only along an upper surface and a lower surface thereof. When the cage is of a cylindrical threaded type, the upper and lower surfaces are curved and essentially designed to engage the portion of the vertebrae where bone is unremoved during boring to create an opening for the device.
When interbody devices of this type are used, it is desirable that the device engage as much surface of bone as possible to provide strength and to reduce the likelihood of subsidence of the device into the bone, resulting from contact pressure of the interbody spacer on an intervertebral surface of a vertebra, since part of the bone is spongy by nature, especially near the center. The remainder of the structure mainly functions to support the two engagement surfaces, unless the device is also used as a cage within which to pack bone fusion material. Because it is also desirable in such structures to maintain weight and volume as low as possible, in order to make the device more compatible with the body, it is also desirable to make the entire device as small and lightweight as possible, while maintaining sufficient strength to prevent catastrophic failure.
As noted above, the mutually facing intervertebral surfaces of an adjacent pair of vertebrae have different characteristics over their areas. Central regions of the surfaces are somewhat spongy, such that there is a tendency of the interbody spacers to subside or sink into the vertebrae in the central regions. In contrast, outer or edge regions of the surfaces are more solid and generally harder. When a fusion cage is implanted, particularly a threaded cylindrical cage, it has previously been the practice to implant two such cages in side-by-side relation except where a wide flat device is used to essentially replace the disc. This done for lateral stability of the vertebrae, so that the vertebrae do not pivot laterally relative to the interbody implant. Two such cylindrical cages have also been used to increase the area of bearing surfaces engaging the vertebral surfaces to thereby minimize subsidence of the cages into the vertebrae. Implanting such a pair of cylindrical cages requires that two bores be cut into the vertebral surfaces to receive the two cages.